It has been accepted practice in the art to provide equipment (commonly referred to as IFF) to distinguish between radar echo signals from friendly aircraft and unidentified, or hostile, aircraft. Thus, various types of equipment have been used to allow an interrogating signal to be transmitted from ancillary equipment at a radar toward a target aircraft and to receive, if such aircraft is friendly, a properly coded reply from a transponder in such aircraft. In order to reduce the IFF equipment which must be carried by friendly aircraft to a minimum and to simplify the necessary ancillary equipment at the radar, a single frequency band has been reserved for IFF systems. While such standardization has allowed satisfactory IFF systems to be designed to operate with radars of any frequency, there are instances in which technical problems cannot be solved with conventional IFF equipment.
One particular problem which is often encountered is that a response from friendly aircraft not meant to have been interrogated may be received. Such a response may occur, if proper measures are not taken, when a friendly aircraft is located in a sidelobe of the IFF interrogator at the radar. One way to suppress a response from a friendly aircraft located in a sidelobe of an IFF interrogator is to transmit an interrogating code which will not cause a response to be generated. For example, if there is a particular difference in power level between portions of the interrogating code as received by a transponder in a friendly aircraft, such transponder may be inhibited from generating a reply. Thus, if a first portion of an interrogating code is transmitted through an omnidirectional antenna with a power level equal to the power level of a corresponding first portion in the first sidelobe of the directional antenna and the remainder of the interrogating signal is transmitted through the directional antenna, simple signal processing circuitry may be incorporated in the transponder in an aircraft to determine whether the aircraft is located in the main lobe or the first sidelobe of the directional antenna. The operation of the transponder will be enabled if the aircraft is in the main lobe or inhibited if in the sidelobe. Another measure which may be taken is to transmit through a directional antenna a first portion of an interrogating code in a sum channel and the second portion of such signal through a difference channel. Again, simple signal processing circuitry in the transponder in an aircraft could be used to determine whether such aircraft is in the main lobe or a sidelobe of the directional antenna. Unfortunately, however, there may be no space available for an omnidirectional antenna or the performance characteristics of a directional antenna transmitting a difference pattern may not be good enough for the particular application.